Supplemental oxygen (hyperoxia) administered to an immature respiratory system following preterm birth is a major risk factor for neonatal lung injury. For survivors of this critical period, the major short-term and long- term health problem remains chronic airway disease manifested as wheezing, asthma, and increased risk of respiratory infection. The long term goal of our studies is to understand mechanisms that contribute to hyperoxia-induced hyperresponsiveness of the immature airway. In previous grant cycles, we established that moderate (50%) hyperoxia in early postnatal development enhances bronchoconstriction, while impairing bronchodilation. The current proposal focuses on mechanisms underlying hyperoxia-induced enhancement of airway tone and constriction. Recent studies in adult airway suggest a role for the neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor TrkB in local control of airway structure and function, especially via effects on airway smooth muscle (ASM). The role of BDNF in developing airway (or with insults such as hyperoxia) is still under investigation. Preliminary studies show that in developing airway, BDNF enhances [Ca2+]i/contractility of fetal ASM. Furthermore, hyperoxia increases BDNF generation/secretion by ASM, and enhances pro-contractile and pro-proliferative effects of BDNF. Accordingly, we believe that BDNF is a key player in hyperoxia-induced increase in tone and reactivity of developing airway. The current proposal focuses on understanding regulation of BDNF and its effects in developing airway. Here, cGMP and cAMP are proposed to normally have opposing effects on BDNF while hyperoxia alters cyclic nucleotide levels such that BDNF levels are allowed to rise. Based on preliminary data, we propose that hyperoxia and BDNF interact to enhance bronchoconstriction via the following mechanisms: 1) Hyperoxia decreases ASM cGMP but increases cAMP, disrupting normal cyclic nucleotide control of BDNF and allowing BDNF production/secretion to rise; 2) via TrkB receptors, increased BDNF has autocrine/paracrine effects of enhancing bronchoconstriction by increasing ASM [Ca2+]i and sensitivity for force generation; 3) separately, BDNF and hyperoxia enhance ASM cell proliferation contributing to airway remodeling; 4) hyperoxia-triggered increase in BDNF serves to initiate and maintain increased airway tone and reactivity. We propose the following Specific Aims: Aim 1: In developing ASM, determine mechanisms by which hyperoxia increases BDNF; Aim 2: In developing ASM, determine role of BDNF in hyperoxia-induced enhancement of contractility; Aim 3: In developing ASM, determine role of BDNF in hyperoxia-induced enhancement of cell proliferation; Aim 4: In a mouse with altered TrkB signaling, determine role of BDNF in neonatal airway hyperresponsiveness. We will use novel in vitro models (human fetal ASM, neonatal mouse bronchial ring and living lung slice) and in vivo neonatal hyperoxia applied to a transgenic TrkB knock-in (TrkB-KI) mouse. The clinical significance of our studies lies in focus on a novel mechanism with therapeutic potential for chronic lung diseases of newborns and beyond.